Moscow explosion: Experts say the explosion, which killed at least 35, will likely be traced to Russia's volatile north Caucasus region, where political murders, bombings, and kidnappings occur almost daily.

Leonid Mlechin, an independent journalist who covers security issues, says the return of terrorism to Moscow shows that Russian security forces have failed to address the lessons of past terrorist attacks.

"This is a serious failure of special services," he says. "No one can carry out such acts alone. He needs to be trained, equipped, and supported by an organized group. Our intelligence services have clearly not been able to catch these groups."

He adds: "Our society just does not recognize the seriousness of the situation in the north Caucasus. There is a knot of problems, and every year they are not dealt with they grow and become more difficult to solve."

Medvedev cancels trip

Russian President Dmitry Medvedev, who postponed a planned trip to the Davos economic forum when the news hit, was quoted by news agencies as saying the perpetrators would be hunted down and punished.

Experts say the explosion will almost certainly be traced to Russia's volatile north Caucasus region, where political murders, bombings, and kidnappings are an almost daily occurrence.

Stability has been gradually collapsing in the mainly Muslim region, but little of the violence gets reported beyond Russian media until a terrorist attack hits a large Russian city, as last happened nearly a year ago when twin suicide bombers hit the crowded Moscow metro at morning rush hour, killing almost 40 people.

Separatist movement has taken on an Islamist thrust

One possible suspect is Doku Umarov, the Chechen warlord who took over the mantle of separatist rebels – who fought two wars for Chechen independence from Russia – and gave it a militant Islamist ideological thrust.

"Nobody should be surprised by this attack, particularly Russian security services," says Andrei Soldatov, editor of Agentura.ru, an online journal that studies the secret services. "The [terrorists'] tactic is to step up activities in the Caucasian republics and bring it to Moscow. Terrorist activities have growing in number in the north Caucasus region," and it was only a matter of time before they struck in Russia's heartland again, he says.

Domodedovo airport, which was shut down by a massive power outage during a late December ice storm, is considered Russia's most modern airport, but is also its busiest. Many experts have warned that it lacks facilities to handle all the traffic it generates.

"Domodedovo is too crowded and has too many routes," says Mr. Soldatov. "We all remember recent problems," he says referencing a power blackout that shut the entire airport down for two days.